1. Field of the Invention
The present invention relates to a method for measuring the twist imparted to an optical fibre. The present invention also relates to a procedure for processing an optical fibre using this method of measurement.
For the purposes of the present invention, the term procedure for processing an optical fibre denotes any process in which an optical fibre is advanced in a predetermined direction, and a twist, the extent of which it would be useful to know, is imparted to it (voluntarily or involuntarily) during its advance. This process may be, for example, a procedure for producing an optical fibre (typically a drawing process) or a procedure for manufacturing an optical cable by a plurality of optical fibres.
2. Description of the Related Art
As is known from “Fiber-Optic Communication System”, by Govind Agrawal, John Wiley and Sons, Inc., Second Edition, a single-mode optical fibre can be used to transmit two orthogonal modes (typically denoted TE and TM) which are degenerate, having the same value of a parameter n known as the mode index (or effective index) and defined as n=β/k0, where β is the constant of propagation and k0 is the free-space wave number. The degenerate nature of the orthogonally polarized modes is present only in an ideal single-mode fibre with a perfectly cylindrical core of uniform diameter. According to the said document, real optical fibres have considerable variations in the shape of the core along their length. They may also undergo non-uniform stresses such that their cylindrical symmetry is disrupted. Because of these factors, the degeneration between the orthogonally polarized modes is removed, and the fibre acquires birefringence. The degree of birefringence is defined as B=|nx−ny|, where nx and ny are the mode indices for the orthogonally polarized modes.
A parameter of particular importance in the study of the birefringence of an optical fibre is what is known as the beat length, which, as described in U.S. Pat. No. 5,418,881 in the name of AT&T Corp., corresponds to the length of fibre required for a given state of polarization to be repeated. In other words, the beat length corresponds to the length of fibre required for the two components of the fundamental mode, initially in phase with each other, come to be in phase again, on the assumption that the fibre maintains a constant birefringence over this length.
Also according to Agrawal, when pulsed signals are transmitted in an optical fibre, the birefringence is a potential cause of the spreading of the pulses. This is because, if an input pulse excites both of the polarization components it becomes wider at the output of the fibre since the two components are dispersed along the fibre owing to their different group velocities. This phenomenon, known as polarization mode dispersion (PMD), has been widely studied in recent years because of its importance in periodically amplified light guide systems.
Typically, the phenomenon of PMD leads to a limitation of the width of the signal transmission band and, consequently, a degradation of the performance of the optical fibres along which the aforesaid signals are transmitted. This phenomenon is therefore undesirable in systems of signal transmission along optical fibres, especially in those operating over long distances, in which it is necessary to minimize any form of attenuation or dispersion of the signals to guarantee high performance in transmission and reception.
The Applicant has observed that the structural and geometrical irregularities of the optical fibre which give rise to birefringence may originate during the process of drawing the fibre. The process of drawing an optical fibre is typically carried out by means of suitable equipment known as a “drawing tower”, starting from a glass preform. In practice, after the preform has been placed in a vertical position and heated to a temperature above the softening point, the molten material is drawn downwards at a controlled velocity in such a way as to produce a threadlike element which forms the optical fibre itself.
An example of a drawing process is described in U.S. Pat. No. 5,298,047 in the name of AT&T Bell Laboratories. In this process, a preform is first advanced inside a furnace and an optical fibre is drawn from a lower neck-shaped (necked down) portion of the preform. The fibre is then made to pass through a diameter monitor and then through a coating applicator, where a polymer coating is applied to the optical fibre which has now cooled. After this, the fibre is made to pass through a coating concentricity monitor, a curing station and a coating diameter monitor. Below this diameter monitor there are drive and guide means, which pull the fibre and guide it towards a take-up spool.
The Applicant has observed that, in a drawing process such as that described above, structural and geometrical defects of the preform, as well as undesired variations of the operating conditions of the process, may result in the presence of birefringence (and consequently of PMD) in the drawn fibre.
The aforesaid U.S. Pat. No. 5,298,047 proposes a method for imparting a torque to the optical fibre during the drawing process, in such a way as to produce a fibre with reduced PMD. This torque is imparted after the coating device, in the direction of advance of the optical fibre. In particular, this torque is imparted by making a guide roller of the optical fibre (forming part of the aforesaid drive and guide means), having its axis perpendicular to the axis of advance of the fibre, move in a suitable way, by alternate oscillations in the clockwise and anticlockwise directions.
Patent application EP 0795521 A1 in the name of Sumitomo Electric Industries relates to an improvement of the technique of U.S. Pat. No. 5,298,047, wherein the guide roller that follows the oscillating guide roller has a V-shaped, U-shaped or convex narrow groove to suppress rolling of the fiber on the surface thereof, which could influence the twist of the fiber, and wherein optical fiber responsive motion suppressing means are provided above the oscillating guide roller to avoid an excessive responsive motion of the fiber.
Patent application EP 0842909 in the name of FOS describes an alternative method of imparting an alternating twist to an optical fibre during the drawing process. This method requires the rotation of the device for applying the protective coating during the drawing process, in such a way that the rotation of this device is transmitted to the optical fibre.
As is known, for example from patent application EP 646819A1 in the name of AT&T Corp., in addition to the birefringence generated in an optical fibre during the drawing process, there may be other causes of birefringence, for example asymmetric stress conditions on the optical fibre caused by external stresses such as those which are present in a cabling process for the production of an optical cable.
As is known, for example from U.S. Pat. No. 4,744,935 in the name of Societa Cavi Pirelli S.p.A., an optical cable for submarine telecommunications may comprise an optical core in which are embedded a plurality of optical fibres for the transmission of optical signals and one or more outer reinforcing and protective coatings. The optical core may be of the type comprising a central supporting element and, around this, one or more layers of polymer material in which the optical fibres are embedded in fixed positions.
As described in patent application EP 646819A1 in the name of AT&T Corp., a process of fabricating the optical core of an optical cable may comprise:                heating a central strength member and extruding a first layer of thermoplastic elastomer onto the heated central strength member;        paying out optical fibres from a plurality of bobbins;        helically laying the optical fibres onto the first layer of thermoplastic elastomer;        passing the central strength member, coated with the first layer of thermoplastic material and wrapped by the optical fibres, through a helically rotating closing die to apply radially inward forces on the cable core, wherein no force is applied to the optical fibres tangentially to the thermoplastic material; and        extruding a second layer of thermoplastic elastomer over the fibers to merge with the first layer.        
The Applicant has observed that the performance in terms of PMD of a cabled optical fibre are generally a function of the changes of shape undergone by the optical fibre as a result of the cabling process. If an optical fibre has undergone a local deformation, there will be a local delay between the orthogonal propagation modes of the signals in the deformed region during the transmission of signals. The summation of the delays introduced along the optical fibre determines the PMD of the signals transmitted.
The method of imparting a twist to an optical fibre to reduce its birefringence can also be applied in a process of cabling the optical fibre to produce an optical cable.
The aforesaid patent application EP 646819A1 proposes a method for reducing the PMD of optical fibres cabled in the way described above. This method requires the imparting of a twist to the optical fibres about their corresponding axes, after they have been unwound from the reels. This twist is imparted by using a wheel mounted rotatably about the central reinforcing element and carrying the reels for unwinding the optical fibres. The wheel rotates simultaneously with the unwinding of the reels, producing a twist in the unwound optical fibres.
The Applicant has observed that the methods described above for reducing the birefringence of an optical fibre during the drawing process and for reducing the birefringence of a set of optical fibres during their cabling has the principal disadvantage that the actual twist which the optical fibre has at the end of the process is different from the theoretical torsion predicted on the basis of the applied torque. In practice, owing to phenomena of friction and the undesired application of non-negligible torques, the optical fibre has locally an actual angle of rotation different from the theoretically predicted angle, and therefore an actual twist different from the theoretically predicted twist. Moreover, the value of this actual twist cannot generally be determined with precision.
In the case of the drawing process, for example, if the twist is imparted after the application of the protective coating, the twist is retarded by the viscous friction which occurs within the coating device (because this viscous friction gives rise to a resistant torque opposing the applied torque). On the other hand, if the twist is imparted to the optical fibre before the application of the polymer protecting material. the optical fibre could be damaged, and its optical properties would therefore deteriorate.
In the patent EP 0842909, in which the torque is imparted to the optical fibre by the rotation of the coating device, the application of the torque takes place in the presence of viscosity (associated with the coating material), and therefore the amplitude of the rotation transmitted to the optical fibre is difficult to control.
A further cause of indeterminacy of the actual twist imparted to the optical fibre during the drawing process is related to the fact that, since the optical fibre is drawn downwards from a heated end of the preform (neckdown) and progressively cooled, the portion of optical fibre which extends from the preform to the twist device at each instant has a non-uniform consistency, owing to these temperature variations. In particular, the optical fibre is substantially consolidated at the twist device, while it is still partially molten at the lower end of the preform (neckdown). The application of a torque to the optical fibre at a point at which it is substantially consolidated (preferably, for the reason explained above, after the coating device) causes the partially viscous transmission of the twist to the less solid portions of fibre, as far as the neckdown. As a result of this, the fibre has, at the neckdown, an angle of rotation which is out of alignment with, and smaller than, the angle of rotation in the consolidated portion. This makes it more difficult to impart the desired twist to the optical fibre.
The Applicant has observed that the decrease in the amplitude of the rotation from the consolidated portion of fibre to the portion of fibre at the neckdown becomes greater as the frequency of the reciprocating motion increases. The Applicant has also noted that, when the methods described in the aforesaid U.S. Pat. Nos. 5,298,047, 5,418,881 and WO 9846536 are used, high frequencies of inversion of the rotation of the optical fibre may cause an undesired slip of the optical fibre with respect to the elements imparting the twist to it. The presence of this slip prevents the imparting of the desired torque to the optical fibre.
The twist actually imparted to an optical fibre in a drawing process is generally monitored by means of destructive tests. In particular, this twist monitoring is carried out by creating artificially a predetermined distribution of defects (for example air bubbles) within the optical fibre before the torque is imparted to it, and then, after the torque has been imparted, by analysing the optical fibre at a number of points under the microscope to check the arrangement of these defects. The actual twist “frozen” into the fibre can be derived from the arrangement of the defects. An operation of this type can advantageously be used before the start of the fibre production process, and requires the use of a portion of fibre which can be sacrificed. However, the Applicant considers that, since the process conditions and parameters are generally subject to variations (desired or undesired), the actual twist imparted during the process may prove to be different from that determined in this way, and its correct determination must be carried out on line during the production process.
Similar considerations are applicable to a process of cabling optical fibres in which the method of imparting a twist to the fibres is used to reduce the birefringence of the optical fibres. For example, the viscous effects present at the interface between the optical fibres and the thermoplastic material on which the fibres are typically deposited make it difficult to monitor the twist which is actually imparted. Further difficulties, according to the Applicant, arise from the fact that, with a method such as that described in the aforesaid patent application EP 646819A1, the actual twist imparted to the optical fibres depends on numerous process parameters, including the velocity of advance of the optical fibres in the direction of processing and the angular velocity of twist, and the undesired variations of any one of these parameters affects the value of the actual twist imparted to the optical fibre and therefore makes it different from the predicted value.
At the present time, the PMD of the different optical fibres in an optical cable can be determined precisely only after the cable itself has been completed, by measuring the PMD in a portion of the optical cable of suitable length.
Given the presence of a discrepancy between the twist which would be imparted to the optical fibre in the absence of disturbing effects (and that can be determined by theoretical calculations on the basis of the applied torque) and the actual twist which the fibre has at the end of the process in question, a method which enabled the actual twist to be determined on line in a simple and precise way would be of considerable use for the purpose of gaining a precise knowledge of the PMD of the fibre.
A technique for controlling the internal rotation of the principal axes of a birefringent optical fibre during the process of fabricating the fibre is proposed in the article “Technique for Controlling the Internal Rotation of Principal Axes in the Fabrication of Birefringent Fibers”, C. G. Askins and M. J. Marrone, Journal of Lightwave Technology, Vol. 6, No 9, September 1988. This technique is based on a method for interpreting the optical power backscattered by a birefringent fibre illuminated laterally in such a way as to determine the internal orientation of the principal axes of birefringence. This technique consists in laterally illuminating the optical fibre with a He—Ne laser beam in such a way as to generate interference fringes by means of the backscattered light. A rotation of the optical fibre about its axis causes a shift of the fringes, owing to the ellipticity of the fibre or any anisotropy of the refractive index due to stresses. It is then possible to determine the rotation of the principal axes of birefringence of the optical fibre by measuring the shift of the fringes.
In the case of birefringent fibres (such as those considered in the cited article), rotations of the fibre cause a decrease of the birefringence and are therefore undesirable, by contrast with the situations considered above. The Applicant has observed that the technique proposed in the article by Askins and Marrone is suitable for the determination of relatively slow rotations of the optical fibre (of the order of 1 revolution/m) on line, and in fibre with high birefringence (with a beat length typically of the order of a few centimeters). According to the Applicant, this technique would not be suitable for the determination of the twist imparted to the optical fibre by one of the techniques described previously for reducing the birefringence, since these twists usually correspond to relatively high rotation velocities (of the order of 10 revolutions/m, for example), and the fibres in question are of the low-birefringence type.
A different technique for detecting the twist applied to a glass fiber is provided by JP 08245232. According to this patent, during the manufacturing of a glass fibre, the width of the glass fibre is measured in one direction using a fibre diameter-measuring device. Since the width of the glass fibre measured from the longitudinal direction changes depending on the direction of the measurement, the degree of twisting of the fibre can be found by examination of the changes in the measured value of the width of the glass fibre. In practice, as the fibre advances, a graph shows the variations of width as a function of the length of the fibre that has passed through a measuring line, and the angle of twist can be derived from this graph. As stated in the patent, this technique requires a maximum value of the width that is no less than 1.01 times the minimum value, in order to allow distinguishing the changes in the measured value of the width of the glass fibre due to twist from the changes imparted in the manufacturing process.